The present disclosure relates generally to the field of electromagnetic well logging for formation evaluation and characterization. More particularly, the disclosure relates to using multiaxial electromagnetic well logging measurements to resolve formation resistivity anisotropy and formation structures such as cross bedding.
Well logging instruments known in the art include electromagnetic induction and electromagnetic propagation (resistivity) instruments. Earlier well logging instruments were run into a wellbore on an armored electrical cable (“wireline”) after the wellbore had been drilled. More recent versions of such wireline instrument are still used extensively. As the demand for information while drilling a wellbore continued to increase, measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools have been developed to meet such demands. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements (measurements of physical parameters) such as resistivity, porosity, NMR relaxation time distributions, among other measurements. MWD and LWD tools often have features in common with wireline tools (e.g., transmitting and receiving antennas, sensors, etc.).
The use of electromagnetic measurements in applications, such as logging while drilling (LWD) and wireline logging applications is well known. LWD electromagnetic measurement techniques may be used to determine subsurface formation resistivity, which, along with formation porosity measurements, may be used to indicate the presence of hydrocarbons in certain formations. Moreover, azimuthally sensitive directional resistivity measurements are known to be used in “pay-zone” steering applications, to provide information upon which wellbore trajectory steering decisions may be made.
Electromagnetic induction or propagation measurements may be inverted using a formation model to obtain various formation parameters, for example and without limitation vertical resistivity, horizontal resistivity, distance to a remote bed, resistivity of the remote bed, dip angle, and the like. One challenge in using directional electromagnetic resistivity measurements, is obtaining a sufficient quantity of data to perform a reliable inversion. The actual formation structure is frequently much more complex than the formation models used in the inversion. The use of full tensor measurements may enable a full tensor of the formation properties to be obtained.
Geologically, cross bedding is understood as referring to sedimentary formations wherein each bed is made up of thin layers inclined with respect to the bedding plane. For example, FIG. 1 shows a photograph of a cross bedded formation taken of the Navajo Sandstone formations in the Zion National Park, Utah, USA.
In the foregoing example, thin layers may appear in measurements as resistivity anisotropy when their thickness is below the instrument's axial resolution. As can be appreciated, cross bedding can form in any environment with a flowing fluid where mobile sediments are carried along the upstream side, then fall down and deposit on the downstream side. Cross bedding phenomena have been found in a variety of sedimentary environments such as fluvial deposits, tidal areas, shallow marine, aeolian dunes, and other environments.
Triaxial induction tools are known in the art to perform electromagnetic (resistivity) logging of formations. More recent triaxial induction tools may be capable of providing full tensor measurements. With full tensor measurements a user may be able to determine not only the formation resistivity, but also the resistivity anisotropy and dip of certain formations. The ability to determine formation dip with triaxial induction measurements relies on their sensitivity to both dip and orientation of resistivity anisotropy. In transversely isotropic formations where the orientation of the resistivity anisotropy is aligned with the bedding dip, the triaxial induction dip is simply the bedding dip. However, when cross bedding develops in some beds the triaxial induction dip becomes a mixture of both the bedding dip and cross bed dip.
The use of computer models is one way to simulate tool response, and simulated too response may be used for interpretation in order to interpret instrument measurement data to obtain formation properties. As an example, early work on modeling triaxial induction tools in the presence of cross bedding is described in, Anderson et al., The Effect of Cross bedding Anisotropy On Induction Tool Response,” SPWLA 39th Annual Logging Symposium, Paper B. However, the foregoing paper does not explicitly describe how the modeling may be applied in a well of arbitrary dip (the paper discloses vertical wells). Further, the forgoing publication does not explicitly discuss decomposition of electromagnetic fields.